A fuel cell is a device which converts chemical energy of a fuel into electrical energy, typically by oxidizing the fuel. In general a fuel cell includes an anode and a cathode fluid flow plate separated by an electrolyte. When fuel is supplied to the anode and oxidant is supplied to the cathode, the electrolyte electrochemically generates a useable electric current which is passed through an external load. The fuel typically supplied is hydrogen and the oxidant typically supplied is oxygen. In such cells, the electrolyte combines the oxygen and hydrogen to form water and to release electrons. The chemical reaction of a fuel cell using hydrogen as the fuel and oxygen as the oxidant is shown in equation (1). EQU H.sub.2 +1/2O.sub.2.fwdarw.H.sub.2 O (1)
This process occurs through two redox or separate half-reactions which occur at the electrodes:
Anode Reaction EQU H.sub.2.fwdarw.2H.sup.+ +2e.sup.- (2)
Cathode Reaction EQU 1/2O.sub.2 +2H.sup.+ +2e.sup.-.fwdarw.H.sub.2 O (3)
In the anode half-reaction, hydrogen is consumed at the fuel cell anode releasing protons and electrons as shown in equation (2). The protons are injected into the fuel cell electrolyte and migrate to the cathode. The electrons travel from the fuel cell anode to cathode through an external electrical load. In the cathode half-reaction, oxygen, electrons from the load, and protons from the electrolyte combine to form water as shown in equation (3). The directional flow of protons, such as from anode to cathode, serves as a basis for labeling an "anode" side and a "cathode" side of the fuel cell.
The anode and cathode fluid flow plates are made of an electrically conductive material, typically metal or compressed carbon, in various sizes and shapes. Fluid flow plates act as current collectors, provide paths for access of the fuels and oxidants to the cell, and provide a path for removal of waste products formed during operation of the cell. Each fuel cell includes a catalyst, such as platinum, for promoting the chemical reaction(s) that take place on the electrodes in the fuel cells. Additionally, the fluid flow plates include a fluid flow field of channels for directing fluids within the cell.
Fluid flow plates are commonly produced by any of a variety of processes. For example, one technique for plate construction, referred to as "monolithic" style, includes compressing carbon powder into a coherent mass which is subjected to high temperature processes to bind the carbon particles together, and to convert a portion of the mass into graphite for improved electrical conductivity. The mass is then cut into slices, which are formed into the fluid flow plates. Typically, each fluid flow plate is subjected to a sealing process (e.g., resin impregnation) in order to decrease gas permeation therethrough and reduce the risk of uncontrolled reactions.
Fuel cells are classified into several types according to the electrolyte used to accommodate ion transfer during operation. Examples of electrolytes include aqueous potassium hydroxide, concentrated phosphoric acid, fused alkali carbonate, stabilized zirconium oxide, and solid polymers, e.g., a solid polymer ion exchange membrane.
An example of a solid polymer ion exchange membrane is a Proton Exchange Membrane (hereinafter "PEM") which is used in fuel cells to convert the chemical energy of hydrogen and oxygen directly into electrical energy. A PEM is a solid polymer electrolyte which when used in a PEM-type fuel cell permits the passage of protons (i.e.,H.sup.+ ions) from the anode side of a fuel cell to the cathode side of the fuel cell while preventing passage of reactant fluids such as hydrogen and oxygen gases.
Typically, a PEM-type fuel cell includes an electrode assembly disposed between two fluid flow plates. The electrode assembly usually includes five components: two gas diffusion layers; two catalysts; and an electrolyte. The electrolyte is located in the middle of the five-component electrode assembly. On one side of the electrolyte (the anode side), a gas diffusion layer (the anode gas diffusion layer) is disposed adjacent the anode layer, and a catalyst (the anode catalyst) is disposed between the anode gas diffusion layer and the electrolyte. On the other side of the electrolyte (the cathode side), a gas diffusion layer (the cathode gas diffusion layer) is disposed adjacent the cathode layer, and a catalyst (the cathode catalyst) is disposed between the cathode gas diffusion layer and the electrolyte.
Several PEM-type fuel cells usually are arranged as a multi-cell assembly or "stack." In a multi-cell stack, multiple single PEM-type cells are connected together in series. The number and arrangement of single cells within a multi-cell assembly are adjusted to increase the overall power output of the fuel cell. Typically, the cells are connected in series with one side of an fluid flow plate acting as the anode for one cell and the other side of the fluid flow plate acting as the cathode for an adjacent cell.
Fluid flow plates also have holes therethrough for alignment and for formation of fluid manifolds that service fluids for the stack. Some of the fluid manifolds distribute fuel (such as hydrogen) and oxidant (such as air or oxygen) to, and remove unused fuel and oxidant as well as product water from, the fluid flow fields of the plates. Additionally, other fluid manifolds circulate coolant. Furthermore, other cooling mechanisms such as cooling plates are commonly installed within the stack between adjacent single cells to remove heat generated during fuel cell operation.
Typically, the PEM works more effectively if it is wet. Conversely, once any area of the PEM dries out, the electrochemical reaction in that area stops. Eventually, the dryness can progressively march across the PEM until the fuel cell fails completely. As a result, the fuel and oxidant fed to each fuel cell are usually humidified, e.g., with steam.